This invention relates to an alkaline based stripper for adherent organic layers and, in particular, to a composition including propylene glycol ether compounds and potassium hydroxide for stripping photoresist.
In the manufacture of semiconductor wafers, and other applications, a layer of metal, semiconductor, or insulator is coated with photoresist. The photoresist is exposed to actinic radiation and then etched in a suitable etchant to produce a pattern in the photoresist. The remaining photoresist protects selected areas of the underlying layer while exposed areas are treated. After treatment, the remaining photoresist must be removed.
Photoresist is chemically altered by exposure to actinic radiation. Specifically, after radiation, the photoresist no longer etches uniformly and the more easily etched material is removed to produce a pattern. Thus, by definition, the material that remains on the layer is more difficult to etch. In principle, one can easily remove any photoresist with a variety of strippers. The problem is that one wants to remove only the photoresist and not etch or corrupt any other material or leave any residue of either the stripper or the photoresist.
A semiconductor wafer may include on one surface thereof exposed silicon, oxides and nitrides of silicon or a metal, and one or more metals, such as copper, aluminum, tungsten, nickel, and gold. A printed circuit board includes many of the same materials. It is difficult to find a stripper that is selective, i.e. that strips or removes photoresist without attacking another part of a processed wafer or a printed circuit board. Obviously, at some level of magnification, a stripper can be shown to have corroded an exposed aluminum layer, for example. What is desired is a minimum level of corrosion, below which further processing is unaffected and the electrical operation of the product is unaffected.
It is known in the art to include a hydroxide in a stripper for photoresist. In aqueous solution, a hydroxide is too vigorous for use as a stripper because many or most of the above-identified materials on a wafer are also attacked by the hydroxide. Several strippers have been developed in which a hydroxide is used in an organic solvent. For example, U.S. Pat. No. 4,744,834 (Haq) discloses a mixture including quaternary ammonium hydroxide, diethylene glycol monoalkyl ether, polyglycol, and 2-pyrrolidone. U.S. Pat. No. 5,529,887 (Horn et al.) discloses a stripper for removing cured photoresist or a cured solder mask. The stripper includes an alkali hydroxide, such as sodium hydroxide, potassium hydroxide, or quaternary ammonium hydroxide, a glycol monoalkyl ether having two to six carbon atoms, and a water soluble fluoride.
Solder masks differ from photoresist masks in at least two respects. A photoresist mask is typically one or two microns thick, whereas a solder mask is typically several microns or tens of microns thick. In addition, a solder mask is exposed to heat after curing; e.g. immersing a circuit board in a solder bath. The subsequent heating further hardens the mask, particularly if the solder mask is heat curable. For these reasons, a solder mask is more difficult to remove than a photoresist mask.
Ethylene glycol monoalkyl ether solutions containing potassium hydroxide and co-solvents effectively remove photoresists and other organic materials, such as residues from solder operations. Unfortunately, ethylene glycol ethers are classified as hazardous air pollutants by the U.S. Government, requiring expensive equipment and procedures for handling and disposal. Further, the most effective ethylene glycol ethers have low boiling points and low flash points. A wafer is typically stripped in a solution at a temperature of 90-110.degree. C., near the boiling point and above the flash point of many ethylene glycol ethers. Thus, strippers based upon this solvent produce volatile organic compounds that must be contained and that pose a fire hazard at operating temperatures. Finally, the most effective of this group of solvents, ethylene glycol monomethyl ether, is known to be highly toxic to human beings.
In view of the foregoing, it is therefore an object of the invention to provide an alkaline organic stripper containing or producing no known hazardous air pollutants.
Another object of the invention is to provide an alkaline organic stripper that does not use solvents known to be toxic to human beings.
A further object of the invention is to provide an alkaline organic stripper having a high flash point (&gt;110.degree. C.)
Another object of the invention is to provide an alkaline organic stripper having a high boiling point (&gt;150.degree. C.).
A further object of the invention is to provide an alkaline organic stripper that is highly selective.
Another object of the invention is to provide an alkaline organic stripper producing less volatile organic compounds than strippers of the prior art.
A further object of the invention is to provide an alkaline organic stripper that can remove photoresist masks and solder masks.